A linear regulator like the 7805 + transistors will simply "burn off" the voltage difference of 12 V - 5 V = 7 V. At the current of 6 A that means about 42 Watts will be converted into heat. That will require a massive heatsink which can be expensive.
You will be much better off using a switched converter which is much more efficient and will be cheaper as well as you do not need the large heatsink anymore.
I would never recommend a beginner to build their own switched converter, the chance that it will simply not work is quite large. Especially at the large current you want, the chance of success will be small as you don't yet know what it takes to make that work.
I'm experienced with electronics and even I don't build my own switched converters. Instead I just buy a module! Much easier and also cheaper.
So I also suggest that you just buy a ready made module, here's an example.
6 A is quite a lot of current, if you need it for multiple devices then I suggest that you split them in groups of about 3 A each and then use two modules to power them. Connect all (black) ground wires together but keep the (red) 5 V wires separate per group. Spreading the load like this will very likely prevent issues in the future. These cheap modules usually don't work well at or near their full load current. So it is better to spread the work across two modules which then operate at a lower current. Often they can handle that much better.